Growth, yield and physiological response of carrot (Daucus carota L.) to different fertilizer levels and bio-stimulants

Abstract

English: Concerns regarding environmental pollution, emanating from agricultural effluent due to abusive use, have led to a paradigm shift in production technology. On the other hand, subsistence farmers in developing countries cannot afford fertilizer at the current prizes. Farmers, therefore, have to reconcile agronomic, economic, agricultural product quality and environmental aspects of crop. In view of these concerns this study evaluated the response of carrot (Daucus carota L. cv. Karina) to varying fertilizer levels, applied singly and in combination with two commercial bio-stimulants, ComCat® and Kelpak®. Both biostimulants are plant extracts, containing natural active compounds, and are applied exogenously to manipulate crop growth and yield. No reports on the use of either of the two products on carrot production could be found in literature. During both seasons of greenhouse studies, the incremental increase of NPK fertilizer contributed to increased vegetative growth and root fresh mass, though not significantly at all times. The application of bio-stimulants had an erratic effect on carrot growth under greenhouse conditions. However, both bio-stimulants in combination with the standard fertilizer level enhanced root fresh mass at harvest (Chapter 3). Under field conditions, especially during the 2007 growing season, foliar application of both bio-stimulants enhanced yield. Although the ensuing increases were not significant, a higher increase of 7 ton ha-1 was attained due to application of a combination of ComCat® with the standard fertilizer. A lower increase of 4 ton ha-1 was achieved with combinations of ComCat® and Kelpak® with the half-standard fertilizer level (Chapter 4). Growth and yield response of tap roots due to application of different fertilizer levels separately and in combination with bio-stimulants was verified through determination of selected physiological activities (Chapter 5). Sucrose content tended to increase, in concert with yield, as the fertilizer levels were incrementally elevated. Conversely, glucose and fructose content decreased in tap roots as maturity was attained, probably due to less sucrose being hydrolyzed at this development stage. ComCat® had a strong enhancing effect on sucrose translocation as evidenced by the positive relationship between sucrose content, radio-active translocation to the roots and final carrot root yield. The latter applied in both instances where the standard and half the standard fertilizer levels were applied in combination with ComCat®. This effect could be related to ComCat® enabling improved sucrose transport across membranes. The effect of Kelpak® on sucrose accumulation and translocation was less evident but, in combination with half of the standard fertilizer, its application led to a slight increase in yield. The accumulation of -carotene, a terpene associated with taste and quality of carrots, was not influenced by fertilizer application. The level, however, increased where ComCat® was combined with the half-standard fertilizer level. To further comprehend the manipulative effects of fertilizer and bio-stimulants on physiological processes influencing growth and yield of carrots, root respiration as well as activities of glycolytic and oxidative pentose phosphate pathway regulatory enzymes was determined. In all cases the activities were only measured at 30% plant development and 24 hours after second bio-stimulant application. Both ComCat® and Kelpak® increased the respiration rate as well as the activity of glycolytic and oxidative pentose phosphate pathway key enzymes during early carrot development under 25% and zero fertilization. The latter was probably due to nutrient stress during early development. However, as carrots matured nutrient acquisition and energy needs were probably reduced as indicated by the respiration rate remaining at a steady state during later development stages and at harvest across all fertilizer levels. This correlated positively with increased sugar levels at maturity and the final yield. In conclusion, the collective interpretation of growth and physiological data acquired during this study at different fertilizer levels, especially when considered in relationship with final yield data at harvest, does not supply a sufficient rationale to recommend either the use of sub-optimal levels on their own or in combination with the two bio-stimulants included in this study for the cultivation of carrots. However, it is recommended that this study be repeated under irrigation conditions but with plots far enough apart to prevent sideways movement of fertilizer and possible contamination of plots.

Afrikaans: Besorgdheid met betrekking tot omgewingsbesoedeling deur landbou afval as gevolg van oormatige gebruik het aanleiding gegee tot ’n paradigmaskuif in produksietegnologie. Aan die anderkant kan bestaansboere in ontwikkellende lande eenvoudig nie bemesting teen die huidige pryse bekostig nie. Boere is dus verplig om agronomiese-, ekonomiese-, produkkwaliteit- en omgewingsaspekte te versoen in hulle produksiestelsel keuse. Hierdie studie is in die lig van hierdie besorgdheid uitgevoer ten einde die respons van geelwortels (Daucus carota L. cv. Karina) op verskillende bemestingspeile, afsonderlik en in kombinasie met twee kommersiële bio-stimulante ComCat® and Kelpak®, te evalueer. Beide bio-stimulante is plantekstrakte met natuurlike aktiewe komponente wat as blaarbespuitings toegedien word om gewasgroei en opbrengs te manipuleer. Geen gepubliseerde verslae oor die gebruik van enige van die twee produkte op geelwortels kon in die literatuur opgespoor word nie. Gedurende beide seisoene van glashuisstudies, het die inkrementele verhoging van NPK bemesting aanleiding gegee tot ’n verhoging in groeitempo en wortelvarsmassa, alhoewel nie in alle gevalle statisties betekenisvol nie. Alhoewel blaarbespuitings met die bio-stimulante ‘n wisselvallige invloed op plantgroei in die glashuis gehad het, het dit in kombinasie met die aanbevole standaard bemesting tot ’n verhoging in oesopbrengs onder glshuistoestande aanleiding gegee (Hoofstuk 3). Onder veldtoestande, veral gedurende die 2007 groeiseisoen, het blaarbespuitings met beide bio-stimulante oesopbrengs verhoog. Alhoewel hierdie verbetering nie statisties betekenisvol was nie, is ’n oesopbrengsverhoging van 7 ton ha-1 gemeet waar ComCat® in kombinasie met die aanbevole standaard bemesting aangewend is. ’n Laer opbrengsverhoging van 4 ton ha-1 is gemeet waar beide ComCat® en Kelpak® in kombinasie met die helfte van die aanbevole bemestingstandaard toegedien is (Hoofstuk 4). Die groei- en oesopbrengsrespons van wortels op verskillende bemestingspeile, afsonderlik en in kombinasie met bio-stimulante, is geverifieer by wyse van geselekteerde fisiologiese aktiwiteite (Hoofstuk 5). Sukrose-inhoud het, net soos oesopbrengs, verhoog namate die bemestingspeile in inkremente verhoog is. Die glukose- en fruktose-inhoud het in teenstelling skerp afgeneem namate die wortels wasdom bereik het, waarskynlik omdat minder sukrose op hierdie onwikkelingstadium gehidroliseer is. Soos aangedui deur die positiewe verwantskap tussen sukrose-inhoud, radio-aktiewe translokasie van suiker na die wortels en die finale oesopbrengs, het blaarbespuitings met ComCat® ‘n sterk verhogingseffek op die translokasie van sukrose gehad. Laasgenoemde is in beide gevalle, waar die standaard en die helfte van die standaard bemesting in kombinasie met ComCat® aangewend is, waargeneem. Hierdie invloed van ComCat® is toegeskryf aan die vermoë daarvan om sukrose translokasie oor membrane te manipuleer. Die invloed van Kelpak® op sukrose akkumulasie en translokasie was minder opvallend, maar in kombinasie met die helfte van die aanbevole standaard bemestingspeil het dit tot ’n redelike verhoging in oesopbrengs aanleiding gegee. Die akkumulasie van -karoteen, ‘n terpeen geassosieer met smaak en kwaliteit van geelwortels, was nie verskillend by verskillende bemestingspeile nie. Maar, waar ComCat® in kombinasie met die helfte van die aanbevole bemestingspeil toegedien is, is die -karoteen vlak aansienlik verhoog. Om die manipuleringseffekte van verskillende bemestingspeile en bio-stimulante op fisiologiese prosesse wat met groei en opbrengs verband hou verder wetenskaplik te begrond, is wortelrespirasie sowel as die aktiwiteite van glikolitiese en oksidatiewe pentosefosfaatweg reguleringsensieme gekwantifiseer. In alle gevalle is die aktiwiteite slegs by die 30% plantontwikkelingstadium gemeet en wel 24 uur na die tweede biostimulant blaarbespuiting. Beide ComCat® and Kelpak® het bygedra tot ‘n verhoging in die respirasietempo sowel as die aktiwiteite van sleutel respiratoriese en oksidatiewe pentosefosfaat ensieme gedurende hierdie ontwikkelingstadium waar geen of 25% van die standaard bemesting toegedien is. Laasgenoemde was waarskynlik die gevolg van ’n stremmingstoestand wat gedurende vroeë wortelontwikkeling geskep is. Namate wortels wasdom bereik het, het die bemesting- en energiebehoeftes van wortels waarskynlik afgeneem soos aangedui deur die bestendige respirasietempo gedurende latere ontwikkelingstadia en by finale oes in die geval van al vier verskillende bemestingspeile. Laasgenoemde het positief gekorrelleer met verhoogde sukrosevlakke en oesopbrengs. Deur hierdie studie is die aanbevole bemestingspeil vir geelwortelverbouing in Suid-Afrika as betroubaar bevestig. Maar, waar 50% van die aanbevole standaard bemesting in kombinasie met beide bio-stimulante toegedien is, is finale oesopbrengste gemeet wat nie betekenisvol verskil het van dit wat met die standaard peil alleen behaal is nie. Vanuit ’n ekonomiese perspektief is tot die gevolgtrekking gekom dat die toediening van laasgenoemde kombinasiebehandeling vir beide bestaans- en kommersiële boere aanbeveel kan word. Ten slotte, die kollektiewe interpretasie van groei- en fisiologiese data wat tydens hierdie studie met verskillende bemestingspeile bekom is, veral in die lig van finale oesopbrengs data, verskaf nie genoegsame rasionaal om die gebruik van suboptimale bemestingspeile, alleen of in kombinase met die twee bio-stimulante wat getoets is, vir geelwortelverbouing aan te beveel nie. Maar, dit word aanbeveel dat hierdie studie onder besproeiingstoestande herhaal word terwyl plotte sodanig gespasieer word dat sydelingse beweging van bemestingstowwe nie tot kontaminasie aanleiding sal gee nie.